The proposed research has the goal of developing and validating interval psychometric scales of visual function limitations and vision disabilities. These interval scales will be developed using Rasch probablistic measurement models applied to ordinal patient rating responses to individual questions. Once developed and validated, these scales will be independent of the particular assessment used, as long as the instrument is calibrated to the scale. The significance of the proposed research is that it will provide a means of estimating measurements of latent functional ability variables for individual patients with visual impairments. In the future, these measurements can be used for parametric studies, epidemiological studies, and clinical outcome studies. The proposed research will identify the number and nature of functional ability scales. It will determine the dependence of those scales on the diagnosis of visual system disorder, the type of visual impairment, the existence of co-morbidities, and the patient's history of rehabilitation. Existing visual function instruments (NEI-VFQ, VF-14, ADVS, and VAQ) and two general function instrument's individual items will be evaluated with respect to scales. To estimate the scales, a large set of specific cognitive and motor activities (e.g., writing a check) will be classified according to functional domain (reading, fine and gross visual-motor, visual information processing [e.g., recognition, localization, orientation], or mobility). In telephone interviews, low vision patients will be asked to rate the difficulty of performing each activity. Rasch analysis will be used to test the hypothesis that there is a global functional ability scale and to test the validity of the a priori visual function domains. Principal component analysis of response residuals will be used to evaluate the dimensionality of visual function limitations. Patients also will be asked to rate the difficulty of achieving specific activity goals (e.g., cook a meal, manage personal finances) and Rasch analysis will be used to estimate a vision disability scale. Item ordering and item intervals on the scales and scale validity will be compared across diagnostic groups (AMD, glaucoma, diabetic retinopathy, RP, CVA, and anterior segment disorders) and for different types of visual impairments (e.g., acuity loss and contracted visual fields). Person measures of functional ability will be evaluated as a function of severity of visual impairments (visual acuity, contrast sensitivity, visual fields, dark adaptation, color vision). Determining if the NEI-VFQ, VF-14, ADVS, VAQ, SF-36, and SIP can be calibrated to common scales will test the hypothesis that there is a common functional ability variable(s).